1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus wherein mechanical oscillations are generated as a result of the operation of the apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet system. The magnetic resonance apparatus also has a radio-frequency system that beams radio-frequency signals into the examination subject for triggering magnetic resonance signals and picks up the resulting magnetic resonance signals from which magnetic resonance images are produced.
For generating gradient fields, suitable currents must be set in gradient coils of the gradient coil system. The amplitudes of the required currents amount to up to several 100 A. The current rise and decay rates can be up to several 100 kA/s. Given a basic magnetic field on the order of magnitude of 1 T, Lorentz forces that lead to oscillations of the gradient coil system act on these time-variable currents in the gradient coils. These oscillations are transmitted to the surface of the magnetic resonance apparatus via various propagation paths. At the surface, the mechanical oscillations are converted into acoustic oscillations that ultimately lead to unwanted noise.
A number of passive and active noise-reduction measures have been proposed for magnetic resonance apparatuses. For example, known passive noise reduction measures include the application of foamed materials for lining components toward the gradient coil system and/or the use of flexible layers at and/or in the gradient coil system. U.S. Pat. No. 4,954,781 discloses examples of such measures.
As an active noise reduction measure, for example, German OS 44 32 747 discloses the use of actuators allocated to the gradient coil system that, in particular, contain piezoelectric elements whose deformation can be controlled such that deformations of the gradient coil system that occur during operation of the magnetic resonance apparatus can be actively countered. The piezoelectric elements are appropriately controlled by means of an electrical voltage applied thereto. The introduction or attachment of a number of piezoelectric components into the gradient coil system, which is comparatively expansive in terms of space, and the voltage supply and the drive arrangement involve a great technological and economic outlay.
An object of the present invention is to provide an improved magnetic resonance apparatus wherein a highly noise-reducing effect can be achieved by means of a simple passive measure.
The above object is achieved in accordance with the principles of the present invention in a magnetic resonance apparatus wherein at least a part of the magnetic resonance apparatus is fashioned of foamed metal for damping the mechanical oscillations which arise during operation of the apparatus.
By forming at least a part of the magnetic resonance apparatus of foamed metal, whose intrinsic properties allow a fashioning of the part as needed for its function while also achieving a high sound absorption, additional noise reduction measures such as initially set forth as examples are superfluous. The part, for example, is a component of the magnetic resonance apparatus that is indispensable for an operation of the magnetic resonance apparatus. The following are particularly relevant as properties of the foamed metal that allow the part to be fashioned according to functional demands: The foamed metal exhibits a high isotropy, as a result whereof no limitations arise due to vectorial, privileged directions given multi-dimensional, complex structures. Relative to its weight, the foamed metal has a high specific rigidity. The foamed metal can be easily and flexibly processed, so free-form surfaces can be produced. Further, the foamed metal is non-combustible and its surface can be upgraded, structured and/or lacquered.
In an embodiment, at least one region of the foamed metal is filled with a substance, so that voids of the foamed metal are at least wetted or filled. The substance is thereby selected such that, for example, a mechanical damping and/or a thermal conductivity of the filled metal foam are set according to correspondingly defined rules. Visco-elastic polymers, for example polyurethane foams, can be utilized for setting the damping. A thermoplastic synthetic, for example, is injected into the metal foam as a substance for reducing the thermal conductivity. In one embodiment, filled and non-filled regions are arranged next to one another such that a directed heat conduction is achieved.
In another embodiment, at least one region of the metal foam is fashioned such that at least one property of the region is variable. For example, the weight of the region and its thermal conductivity as well are variable by undertaking a local variation of a metal structure density of the metal foam. Further, an increased acoustic absorption of the region can be achieved by fashioning the region with Helmholtz resonators that, for example, are fashioned a depressions in the shape of a truncated pyramid that proceed from a surface of the metal foam. The smaller of the end faces of a truncated pyramid thereby forms an opening of a depression that is accessible proceeding from the surface. The effect of the depression is that an acoustic wave front enters into the depression through the opening, is multiply reflected in the depression and thereby loses intensity. In another embodiment, the Helmholtz resonators are integrated into the metal foam a aperture-free chambers.
In an embodiment, a surface of the metal foam is fashioned in an open-pore manner. As a result of the open-pore fashioning of the surface of the aluminum foam, the sound absorption can be enhanced further compared to a closed-pore fashioning, similar to the result of the Helmholtz resonators. By contrast, a smooth surface or a low porosity produces a high reflectivity, so that only a small part of an acoustic power is absorbed in the metal foam.
In another embodiment, the metal foam is aluminum foam that, for example, is offered under the trademark ALULIGHT(copyright) by Alulight International GmbH in Ranshofen, Austria.